Automatic volume control system



July 6, 1937; R. D. BROWN, JR 2,086,465

AUTOMATIC VOLUME CONTROL SYSTEM Filed March 3, 1952 Patented July 6,1937 PATENT OFFICE AUTOMATIC VOLUME CONTROL SYSTEM Reynolds D. Brown, Jr., Philadelphia, Pa., assignor to Philadelphia Storage Battery Company,Philadelphia, Pa., a corporation of Pennsylvania Application March 3,1932, Serial No. 596,599

12 Claims.

This invention relates tocarrier waves receivmg systems and hasparticular reference to the automatic control of the volume of theoutput of such systems. The present invention is related to the subjectmatter of co-pending application of David G. McCaa and Reynolds D.Brown, Jr., Serial No. 405,964, filed Nov. 9, 1929, Patent No.2,061,710, November 24, 1936 and that of copending application ofReynolds D. Brown, Jr., Serial No. 412,163, filed Dec. 6, 1929, PatentNo.

1,988,370, January 15, 1935.

An object of the invention is to provide means for limiting the outputvolume of a system to a certain predetermined level regardless ofincreases in the amplitude of the carrier wave input to the system.

Another object of the invention is to provide means for so controllingthe output volume of a radio-receivingsystem that strong carrier wavesare=reduced to a predetermined level while carrier waves which areweaker than that level will be only slightly affected.

A further and more specific object of the invention is to provide asystem wherein a uni-directional control voltage corresponding inamplitude'to the amplitude of the carrier voltage is derived from theinput or grid-cathode circuit of a thermionic or vacuum tube detector ofthe system. Otherwise stated, the invention contemplates the use of thegrid rectification current of the detector to derive the uni-directionalcontrol voltage, such voltage being applied directly to the control gridor grids of one or more of the radio frequency amplifiers to therebycontrol the efiiciency of such amplifiers.

Afurther object of the invention is to provide a system wherein adetector circuit having high amplification and overload characteristicsis utilized/as a source of control energy for con- 40 trolling the radiofrequency amplifiers.

A still further object of the invention is to provide a system'whereingrid-leak detection is made stage is shown as including a screen gridtube quency currents having useful overload characteristics.

A clear understanding of the invention may be had from the followingdetailed description and the accompanying drawing, wherein there isillustrated a specific embodiment of the invention.

In the drawing:

Fig. 1 is a schematic circuit diagram of a simple receiving systemembodying certain features of the invention; and 10 Figs. 2 to 5 aresimilar diagrams of modifications.

Referring to Fig. 1 of the drawing, there is illustrated a receivingsystem having the usual antenna I connected through the primary windingof an input transformer T to ground as at 2. Transformer T serves tocouple the antenna circuit to a tuned radio frequency amplifier stagecomprising a radio frequency vacuum tube amplifier V. The usual tuningcondenser 3 is connected across the output of transformer T, and 0 theusual energizing sources, herein illustrated as batteries 4 and 5, areprovided for amplifier V.

In the present illustration, a single stage of radio frequencyamplification is shown but this is merely for the purposes ofillustration and clarity, it being understood that any desired number ofradio frequency or intermediate frequency amplifying stages may be used.The stage or stages of high frequency amplification may take anyconventional form and may include as elements thereof any desired typeof potential controlled amplifying device. In the present illustration,the single radio frequency amplifying which comprises, as is well known,a shielding grid in addition to the usual cathode, control grid, andanode. Radio frequency by-pass condenser 6 and filter resistance 1 areincluded in the circuit of the shielding grid in the usual manner.

A second radio frequency transformer T1 serves to couple the amplifyingstage just described to the input of a vacuum tube detector D of theusual type. While shown herein as a three-electrode tube, detector D maytake the form of-any conventional tube which, may be utilized fordetecting the incoming modulated carrier waves. A radio frequencyby-pass condenser 8 and a .choke coil 9 are preferably connected in theoutput circuit of the detector in the usual manner. The filament andanode energizing sources I0 and II, respectively, are provided for theusual purpose and may be includedinsources 4 and 5. An audio frequencytransformer T2 may be utilized to couple the outputcircuit of thedetector to any desired load circuit. The detector may feed one or moreaudio frequency amplifying stages, as is well'known, or it may work intoany desired impedance.

In accordance with the present invention I provide in the grid-cathodeor input circuit of detector D, a resistor R and a condenser Cin shunttherewith. These elements comprise a time circuit, as is wellunderstood, and the inherent time lag action thereof may be given anydesired value by assigning proper. predetermined values to theresistance and condenser. One extremity of the parallel circuitincluding the el ments just mentioned is connected by conductor l2through filter resistor I 3 to one terminal of the secondary winding oftransform-er T. A radio frequency by-pass and filter condenser i4. isconnected between the point of connection of conductor l2 and the groundconnection oi theinput circuit-oftube ,V. Filter resistor l3 functionsin conjunction with condenser C, resistor R and condenser 14 tofilterout any alternating currents in the control circuit. The purpose of thearrangement just described is, aspreviously indicated to derive auni-directionalcontrol voltage I whose amplitude is proportional tothatof the incoming carrier wave and apply such control voltage to theradio frequency amplifier or amplifiers ahead of the detector. v Itwillbe noted thatnonormal biasing voltage is provided for the control gridof detectorD,

which device operates in the manner of the usual grid-leak detector byvirtue of the elements utilizedherein for control purposes. The mannerof operation of a grid-leak detector is well known and it is. deemedunnecessary to describe such action herein except as it is affected bythe present invention. Since no biasing potential is .applied to the.grid of the detector, this element, will remain at substantially zeropotentialwith re-'.,

spect to the cathode during inoperative periods 1.

when no carrier wave isbeing received. Practically no current flow takesplace in the grid-' cathode circuit of the detector at this time andonly a very small voltage is established across resistor R and,therefore, substantially no con trol voltage is applied to the controlgrid of tube V.

Assuming now that a carrier wave is present and excites the system, tubeV, beingsubstantially unbiased, will initially operate in the usualmanner over a portion of its characteristic curve to give the desiredhigh amplification of the incoming carrier Wave. During one-half cycleof the carrier wave, when the potential across the secondary oftransformer T1 has the polarity indicated 1 on the drawing and the gridof tube Dis positively charged, the grid will attract electrons emittedfrom the filament or cathode and grid rectification current will fiow inthe input circuit of detector tube through resistor R: The voltage setup across the resistor by this current flow will have the polarityindicated on the drawing so that a negative control orbiasing potentialwill i be applied to the control grid of tube V to effect the desiredefiiciency change thereof in accordance with the amplitude of thecarrier. 'During the first few or initial cycles of the incoming carrierwave, condenser C willbe gradually charged until the peak voltage oftheapplied wave is reached and this condenser will discharge slightlythrough resistor R during the alternate half cycles when the voltageapplied to the control vl fication current will flow in the inputcircuit of tube D, causing aproportionate increase in the controlvoltage set up across resistor R. This increased biasing voltage beingapplied to tube V will decrease its efficiency and cause a decrease inthe amplitude of the carrier Wave applied to tube D.

In Fig. 2, there is illustrated a modified system, it being understoodthat the portion not illustrated is similar to Fig. 1. In this system aspecially constructed screen grid tube .is utilized as a grid-leakdetector. The use of a screen grid tube as a detector is highlydesirable, as is well known, because of the high audioamplificationwhich is had with this type of tube. The fact that such atube overloads easily when used as .a grid-leak detector has, however,precluded, its use in this manner. In accordance with the'embodiment ofthe invention illustrated in Fig. 2, I provide means which enablesthe-practical use of a screen grid tube as a grid-lead detector, therebyobtaining the advantages inherent in this type of tube as well as in theparticular manner of detection. I I

In order to adapt the screen gridtube to gridleak detection, I providean auxiliary grid l5-in the tube, which grid is interposed between thescreen grid and the anode or plate and is connected, as.

at IE, to the control grid conductor. Aside from this modification, thetube takes the formofthe usual screen grid tube, the construction ofwhich is now well known. As is usual, the screen grid is.

connected to a suitable point of the anode supply source to provide thedesired potential on the screen grid. Suitable provision must be madefor insuring that a path which has a low impedance to audio frequencycurrents exists between the screen and cathode. A 0.25-micro-faradcondenser may be used in this circuit. I have found that by modifyingthe tube in the manner illustrated and described, overloading isreduced-and.

high gain is had. The auxiliary grid constitutes a second control grid,as is obvious from its manner ofconnection in the circuit; Most of thegrid amplifying action however is. had by the main control grid disposedadjacentthe cathode, .and the auxiliary grid merely assists the maincontrol grid in its detectionaof the modulated. carrier wave.. By virtueof its position, however,

the auxiliary grid influences the plate ofanoden.

in such a manner as to prevent the overloading action which is inherentin the ordinary screen. grid tube when used as contemplated herein.

Radio frequency choke l8 may be inserted in series with the grid leadasshown, in order that this grid may be influenced chiefly by audiofre-,quency voltages built up across RC. Thesystem of Fig. 2 operates in thesame general manner as described above in connection with Fig. l insofaras the principles of automatic volume control are concerned. r I

In Fig. 3, there is illustrated afurthermodi fied system embodying theprinciples of the invention. In this system, the useof ascreentubea'fsas a grid-leak detector is made, possible by a different modificationthereof from that of Fig. 2,

and the tube is made to operate as a highly sen-,

sitive amplifier of the audio frequency currents 3 inaddition to itsrectification action. In this embodiment, the main control grid ispositioned between the screen grid and the plate 'or anodev instead ofin its usual position adjacent the cathode as in the ordinary screengrid tube. Furthermore, there is provided an auxiliary grid I! which ispositioned between the cathode andscreen grid to take the place of theusual control grid andconnected to the input circuit at one extremity ofresistor R, as illustrated. This auxiliary grid may be considered as anauxiliary control grid which functions to apply the audio' frequencyvoltage appearing across resistor R to the tube which in turn functionsas a sensitive amplifier to amplify such voltage after the control gridhas rectified or demodulated the incoming carrier wave. The main controlgrid serves, in conjunction with the cathode, to rectify the incomingcarrier wave, these two electrodes of the tube constituting a simpletwo-electrode valve or rectifier. The grid rectification currentsflowing through resistor R are utilized to derive the desiredunidirectional control voltage, as hereinbefore described, and the audiofrequency voltages appearing across resistor R as a result of the audiofrequency component of the rectified currents flowing therethrough, areapplied between the cathode and the auxiliary grid ll, which elementsconstitute the input of the device in its function as an audio frequencyamplifier. The amplified audio frequency currents flowing in the anodeor output circuit may be transferred by a transformer to the loadcircuit. An important practical advantage of the circuit of Fig. 3 isthat three-element detectors of all the well known types are oftenoverloaded because the radio frequency voltage applied to the grid isamplified as muchas by the audio frequency voltage, and with lowpercentages of modulation the radio frequency voltage may overload thedetector plate circuit long before the audio frequency output voltage isof sufiioient intensity. In this instance, the radio frequency isapplied to a grid of low amplification for rectification, whereas theresultant audio frequency voltage is applied to a grid of highamplification. The two actions assist each other to provide efficientdetection.

Aside from the operation just described, this modified form of theinvention functions in the same manner as described above in connectionwith Fig. l. The principles utilized in the system of Fig. 1 to deriveand apply the uni-directional control voltage are utilized in the systemof Fig. 3.

The circuit of Fig. 4 is similar tothat of Fig. 3, except that filtercondenser l9, and filter resistor 20 are provided so that the audiofrequency energy applied to the control grid II will be substantiallyfree of radio frequency energy. The advantage of this arrangement isthat less overload will occur.

:In the modified system. of Fig. 5, the use of a screen grid tube as agrid-leak detector is made possible by providing an auxiliary grid 2|between the screen grid and the anode, such auxiliary grid beingconnected as at 22 to the filament or cathode at a point which-may bemidway between the extremities of this element.

-Aside from this modification, the tube takes the form of the usualscreen grid tube. The auxiliary grid, of course, has the same potentialas the point of the cathode to which it is connected. The effect of thisgrid is to influence the plate or anode in such a manner as to minimizesecondary electron emission from the plate which is possible in theordinary screen grid tube when used as contemplated herein. The use ofan auxiliary grid next to the anode, maintained at a potential near thatof the cathode serves to increase the overload properties of the tube,as is well understood in the art relating to five-element tubes.

It is important to note that a useful action can be obtained from manymulti-grid tubes, wherein one grid-cathode circuit is used inconjunction with a time circuit to provide rectified voltages includinguni-directional and audio frequency voltages, said rectified voltagesbeing applied to another of said grids to amplify said voltages. Theterms main control grid, and auxiliary control grid as used herein mustbe read in the light of this statement, and applied accordingly. I 1

".It is important to note that in each of the systems disclosed herein,condenser C functions as the usual grid condenser of the grid-leakdetector and resistor R serves as the usual grid-leak to relieve thegrid vof accumulated electrons between wave trains. The detector circuitutilized has the inherent high gain of grid-leak detectors and inaddition provides a substantial amount of control energy which may beutilized to control the carrier wave output volume from the radiofrequency amplifier tube in the manner described. By means of theinvention, the advantages of grid-leak detection are had and thedetector is made to serve additionally as a source of controlv energy insomewhat the same manner as in the well-known automatic :volume controlsystem using a two-element rectifier. Where two or more radio frequencyamplifiers are used, the application of the derived uni-directionalcontrol voltage tomore than one of such amplifiers will obviouslyenhance the automatic volume control action.

It is to be noted that the cathodes of all the tubes may be returned toground potential, and that there is no necessity for balancing a batteryvoltage against a potential drop in a re-' sistor. The tubes may all besupplied from the same source of plate circuit energization. These areimportant features, clue to variation of the characteristics of tubes inservice. Theseadvantages have not heretofore been obtainable in systemshaving multi-element detectors using automatic volume control.

Although specific circuit arrangements are disclosed for the purpose ofillustration, any changes in the details thereof, the essential featuresof the invention remaining undisturbed, are deemed to be'within thescope of the invention. Only such limitations as are included within theappended claims are to limit the invention.

I claim:

1. In a carrier wave receiving system, one or more carrier frequencyamplifying tubes, 2. detector tube having a cathode, an anode, a maincontrol grid,and an auxiliary control grid, means for applying theamplified carrier frequency signal voltage between one of said grids andsaid cathode, means for deriving uni-directional and audio frequencyvoltages from the input circuit of said detector tube, the value of saiduni-direc-' tional voltage depending on the carrier voltage,

means for applying said uni-directional voltage to at least one ofsaidcarrier frequency amplifying tubes, whereby the efficiency of saidamplifying tubes is modified in accordance with the carrier waveamplitude, and means for applying said audio frequency voltage betweenanother one of said grids and said cathode.

2. In a carrier wave receiving system, one or more carrier frequencyamplifying tubes, a detector tube having a cathode, an anode, a screengrid, a main control grid interposed between said screen grid and saidanode, and an auxiliary control grid interposed between said cathode andsaid screen grid, means for applying the amplified carrier frequencysignal voltage between said main control grid and said cathode, meansfor deriving uni-directional and audio frequency voltages from the inputcircuit of said detector tube, the value of said uni-directional voltagedepending on the carrier voltage, means for applying saiduni-directional voltage to at least one of said carrier frequencyamplifying tubes, whereby the-eificiency of said amplifying tube ismodified in accordance with the carrier wave amplitude, and means forapplying said audio frequency voltage between said auxiliary controlgrid and said cathode.

3. In a carrier wave receiving system, one or more carrier frequencyamplifying tubes, a de tector tube having a cathode, an anode, and atleast two auxiliary electrodes for controlling the anode current; one ofsaid auxiliary electrodes being arranged to control the anode current toa greater extent .than another of said auxiliary -electrodes, means forapplying a modulated carrier wave to said other auxiliary electrode fordemodulation, and means for applying demodulated signal potentials tothe first-mentioned auxiliary electrode.

4. In a carrier wave receiving system, one or more carrier frequencyamplifying tubes, a detector tube having a cathode, an anode, and atleast'two auxiliary electrodes for controlling the anode current, one ofsaid auxiliary electrodes beingarranged to control the anode currentto agreater extent than another of said auxiliary electrodes, meansforapplying a modulated carrier wave to said otherv auxiliary electrodefor demodulation, means for deriving demodulated 1 signal potentials anda uni-directional voltage as a result of demodulation, the value of saidvoltage depending upon the carrier voltage, means for applying saidsignal potentials to the firstmentioned auxiliary electrode, and meansfor applying said uni-directional voltage to at least one of the carrierfrequency amplifying tubes, whereby the eificiency of said'tubes ismodified in accordance with the carrier wave amplitude.

5. In a carrier wave receiving system, one or .more carrier frequencyamplifying tubes, a detector tube having a cathode, an anode, a maincontrol grid interposed between said cathode and said anode, and anauxiliary control grid interposed between said cathode and said maincon- .trol grid, means for applying the amplified carrier frequencysignal voltage between said main control grid and said cathode, meansfor deriving unidirectional and audio frequency voltages from the inputcircuit of said detector tube, the value of said unidirectional voltagedepending on the carrier voltage, means for applying said unidierectional voltage to at least one of said carrier frequency amplifyingtubes, whereby the efficiency of said amplifying tube is modified in ac-.cordance with the carrier wave'amplitude, and

means for applying'said audio frequency voltage between said auxiliarycontrol grid and said cathode. a

6. In a carrier wave receiving system, one or more carrier frequencyamplifying tubes, a detector. .tube having .iacathode, an anode, a maincontrol grid interposed between said cathode and said anode, .and anauxiliary control grid interposed between said cathode and said maincontrol grid, whereby said auxiliary control grid is arranged to controlthe anode current to a greater extent than said main control grid, meansfor applying a modulated carrier wave to said main control gridfordemodulation,and means for applying demodulatedsignal potentials to saidauxiliary control gridnv 1 7. In a carrier wave receiving system, one ormore carrier frequency amplifying tubes, a detector tube having acathode, an anode, a screen grid, a main control .gridinterposed betweensaid screen grid and said anode, and an auxiliary control gridinterposed between said cathode and said screen grid, whereby saidauxiliary control grid is'arranged to control the anode current to agreater extent than saidmain control grid, means for applying'amodulated'carrier wave to saidmain control gridxfor demodulation, and

means for applying demodulated signal potentials to said auxiliarycontrol grid.

8. In a car-rier wave receiving system, one or more carrier frequencyamplifying tubes, a detector tube having a cathode, an anode, a main'controlgrid, and an auxiliary control grid, means for applying theamplified carrier frequency signal voltage between one of said grids andsaid cathode, means for deriving unidirectional and audio frequencyvoltages from the input. circuit of said detector tube, the Value ofsaid unidirectional voltagedepending on the carrier voltage,

at least one of said carrier frequency amplifying means for applyingsaid. unidirectional voltage to tubes,- whereby the efficiency of: saidamplifying tubes is modified in accordance with the carrier 1.

said other auxiliary electrode fordemodulation,

means for filtering the demodulated signal potentials, and means forapplying the filtered signal potentials to the first-mentioned auxiliaryel ec 10. In a radio receiving system, an amplifying tubeoperatingathigh frequency, a detector tube.

having a cathode, an anode, and a plurality of grids disposed betweensaid cathode and said anode to control the flowof spacefcurrenttherebe-' tween, '2. high frequency input circuit for said;

detector includingsaid Cathode and one of said grids, meansfor'supplying high 'frequencysign'al energy from said amplifier to saidinput circuit,

an audio frequency output circuit for said detectorincluding saidcathbdeand said anode, means for deriving-from said input circuit aunidirectional control'voltage whose value depends on the amplitude ofthe incoming signal, means for applyingsaid voltage to said amplifier tocontrol the gain thereof, and means for applying demodulated signalpotentials to another of said grids. 11. In a radio receiver, thecombination with a carrier wave amplifier, of a detector tube having acathode, a main control grid, a positively polarized output anode, andan auxiliary control grid, an input circuit for said tube upon which thecarrier wave output of said amplifier is impressed, said input circuitbeing connected be- 10 tween the cathode and said main control grid andincluding, in series, a carrier-wave coupling element and a resistorshunted by a condenser, means grounding said tube cathode, a connectionbetween said auxiliary control grid and the junc- 15 tion of saidresistor and. said coupling element for impressing the audio frequencypotential component of demodulated carrier Waves upon said auxiliarycontrol grid, and an audio frequency output circuit connected betweenthe said 20 cathode and said output anode.

12. In combination with a radio frequency signal amplifier of the typeincluding at least one tube provided with a gain control electrode, adetector stage following the amplifier and including a tube providedwith a cathode, an anode, and at least two space current controlelectrodes, an audio frequency output circuit connected between saidcathode and said anode, a signal input network connected between one ofsaid control electrodes and said cathode, the other of said controlelectrodes being connected to said input network, both said controlelectrodes being connected to the high alternating current potentialside of said network, a resistor in circuit with at least said onecontrol electrode and said cathode for developing a direct current gaincontrol bias for said amplifier tube, and a direct current connectionbetween a point of negative potential on said resistor and the said gaincontrol electrode.

REYNOLDS D. BROWN, JR.

